A wide variety electronic circuits including integrated circuits may include a power down circuit. For example, a power down circuit may be used to remove electrical power from an electronic circuit when it is not in use. A power down circuit may be useful for extending battery life in a system as well as for temperature control in a system.
A power down circuit may employ a switching transistor having a common-drain configuration in which a load circuit is coupled to the source of the switching transistor. A power control signal having two possible logic states may be applied to the gate of the switching transistor to control the on/off states of the load circuit. For example, a high logic level of the power control signal may cause a high voltage level at the source of the switching transistor, thereby powering on the load circuit whereas a low logic level of the power control signal may cause a low voltage level at the source of the switching transistor, thereby powering off the load circuit
A power down circuit that employs a switching transistor having a common-drain configuration may have a negative impact on the desired operation of a load circuit. For example, the magnitude of the voltage at the source of a switching transistor having a common-drain configuration depends on the magnitude of the voltage applied to the gate of the switching transistor because it operates as a source follower. As a consequence, the amount of voltage supplied to a load circuit in the power on state depends on the voltage level of the high logic state of the power control signal. Unfortunately, the voltage level of the high logic state may vary significantly in digital circuitry and the variation may cause variation in the operation of a load circuit. For example, the gain of an amplifier in a load circuit may vary in response to variation in its supply voltage. In addition, a variation in the voltage level of the low logic state of the power control signal may prevent a power down circuit from switching completely off.
One prior method for reducing the dependence of a power down circuit on the logic states of a power control signal is to amplify the power control signal before it is applied to the switching transistor. Unfortunately, an amplifier for the power control signal may consume electrical power in the off state, thereby defeating the purpose of a power down circuit.